Field of the Invention
The apparatus and process of the present invention relate to filtration of fluids and more specifically to filtering or straining of fluids which are found in the paper making industry.
Paper is made of cellulose fibers. The primary source of cellulose fibers is the wood of trees. The first step in the paper making process is referred to as pulping. Pulping consists of breaking wood into its components, cellulose and lignin, either by mechanical means, such as grinding in which case the resulting pulp is referred as ground wood pulp.
The chemical pulping process consists of cooking wood chips in a pressure cooker at approximately 100 psi pressure for a period of about one hour to dissolve the lignin. Cooking liquors include sodium hydroxide and sodium sulfide in the kraft process or a liquor calcium bisulfite solution in the sulfite process. After the wood is cooked, it is blown from the digestor resulting in a pulp mass. This mass is then washed to remove the residual cooking liquors and chemicals. If the pulp is to be used to make white paper, for example, for writing or printing, the pulp is then bleached using chemicals such as chlorine and chlorine dioxide or calcium hypochlorite to bring the whiteness of the pulp to a suitable degree. In all of the pulping, bleaching and washing processes, vacuum filters are used with an attempt to retain the pulp and wash out the undesirable materials. A considerable amount of the chemicals and finer fiber particles pass through these washer filter screens and contend to be problems in the process.
Ground pulp is generally made from spruce wood and is principly used in making newsprint paper. It is usually mixed with some chemically prepared pulp so that the paper may have greater strength and resist tearing in handling.
The desirable pulp product is then converted into paper by first mechanically treating it in order to effect the end product-paper-strength properties. The paper making process consists of slurrying the pulp in a suspension of approximately 1 part pulp fiber to 200 parts water then discharging this slurry onto a rotating fine screen continuous endless belt, referred to as a forming fabric. Vibration is applied to the screen to spread the material in a thin uniform layer. The water drains through the fabric and the fibers are retained to form a sheet which is subsequently pressed through ring-type rollers and steam dried to remove water.
It is apparent that when the soft mass is carried by the screen, not all of the fiber and other solid materials can be retained by the forming wire and the smaller particles will penetrate through the wire mesh along with the water which is drained. As a result, the so called "white water" is collected during the sheet forming process and is recirculated in the overall system. A large part of the "white water" is added directly to the stuff to be recirculated, while a small portion of it goes into a save-all device which intends to recover much of the solids from the "white water".
In the paper making process, varous additives including dyes and also pigments such as kaolin clay, calcium carbonate and titanium dioxide are added to give the paper the needed end product properties for printing and writing. All of these additive materials are very expensive and recovery of them is economically attractive. Moreoever, these additive materials, if not recovered, constitute an excess cost of effluent treatment. Thus, the paper industry faces a number of problems which should lend themselves to filtration devices or other means of separating liquid and solids. In some cases, the desirable effect is merely to clean the water so that it can be reused or to decrease the amount of water required in order to make a ton of paper. In other cases, the economic or technical motivation is to save or retain the fiber and non-fibrous paper making raw materials to obtain economic benefit or to minimize the problems with the need to remove them from process waters in an effluent treatment plant to prevent pollution.
Over the past several decades, a number of devices, commonly referred to as "save-alls" have been used. There are three basic types of save-alls: the filtration-type save-all is either a gravity or vacuum-aided filter, wherein the white water from the mill is run across a filter and an attempt is used to retain the fiber and filler or non-fibrous materials wherein the water flows through and is clarified. The second type of save-all is a flotation-type save-all. In this case, water is premixed with a flocculent and/or adhesive which is most commonly a polyelectrolyte synthetic polymer. The polymer flocculated material is charged with air in order to make it float and the save-all device consists of a skimming device to take the floating mass of pulp fibers and other non-fibrous materials off, whereas the clearer water settles and is taken off by a well-type device. The third type of save-all is a sedimentation save-all or clarifier, wherein white water treated with a flocculent is allowed to settle and the clearer material is taken off of the top and the richer settled material is taken and returned to the process off of the bottom. All three types of save-alls have generally shown capabilities in a normal operating system when properly designed to clarify water to approximately 10-20 parts per million.
Major problems occur with process upsets which will overload the save-all and cause the water outflow from the save-all to contain as much as 60-100 parts per million of fiber and water. This places severe restrictions on how the water may be used. For example, water with the type of contaminant that has filler material in it cannot be used as water on pumps or agitators because the inclusion of the abrasive material would ruin the equipment. Water with more than about 10 parts per million of suspended solids is not practical for use in machine showers and other applications because it would plug the showers and cause the process an economic loss to the paper making proces. It has been well known for a number of years that the industry has had a need for a reliable device to consistently clarify water down to levels of 10 parts per million or less of suspended solids to remove or stop the inclusion of filler materials which are abrasive and prevent the water from being re-used and to save the fiber and get its economic value.
In a typical mill running 500 tons a day of paper, the amount of fiber and filler that may be lost to the sewer and subsequently need to be treated may run 5 to 15 tons a day in common application, in a system that is using the best available save-all devices.
Water is the medium which is mostly used in the paper industry to convey the pulp and its various additives. In many cases, as much as 200 times as much water is used as fiber and the necessity to recover the most amount of water and make it suitable for recirculation becomes more important in today's industry.
In addition to the necessity to save the fiber, chemical additives and water, one more aspect must be considered: environmental impact of disposal of the water derived from the paper making process.
As was indicated above, the currently used save-all devices do not accomplish the purpose of complete cleaning of white water with a suitable separation of fiber, other solids and water.
It is therefore a customary practice to place another filtration device downstream from the save-all devices, also with the purpose of recovering as much fiber and solids as possible, thereby rendering the water reclaimed from the filtering apparatus more suitable for recirculation.
Additionally, a number of chemicals which are utilized in the paper making process are costly and their reclamation becomes important in view of the quantity of the chemicals added to the pulp and then screened away as white water.
Over the years, a number of devices have been used which are capable of clarifying water to the required level. For example, a common plate and frame filter could be used. However, it is not economically feasible to use this device because of the required amount of man power and the frequency of cleaning required. Other attempts have been used to use various types of pre-coat filters. While these are satisfactory in filtration, there has never been a proven reliable device that does not require manual cleaning usually on a basis of at least once or twice a day and sometimes as often as once an hour. While these devices would be technologically feasible, the required frequency of cleaning have made them economically unfeasible. Another problem with the requirement manual cleaning of filters is the fact that it is common for the process waters and paper mills to run relatively hot, for example, the 150.degree.-180.degree. F. range, there have been numerous instances of injuries in the industry including severe scalding and burning when employees have been required to manually clean these filters. In most cases, these lost time accidents have led to decisions to discontinue the use of the filter.
Various tube and cartridge type filters can be used. Although they are technologically feasible, they are not economically attractive because of the necessity of cleaning them manually on a peridic basis and the potential exposure to dangerous safety conditions.
During the filtering operation, an additional aspect comes into consideration: cleaning the filtering or straining devices after a filtration cycle has been completed. The currently used technique provides for backflushing of filter or strainer elements by simply reversing the flow of fluid through the filter element. This method does not allow for suitable cleaning of the filter element, since a high level of contaminants is deposited on the exterior of the filter and simple backflushing by reversing of flow does not dislodge the settled cake.
Another problem exists during straining. When straining the white water through 0.0004 (100 microns) slots, a considerable amount of fiber passes through the "clean side". When backflushing with this water, some of this fiber becomes lodged on the inside of the screens on conventional designs.
It is therefore clearly seen that there exists a necessity to provide an efficient device for filtering fluids, such as white water, and reclaiming substantially all fiber solids and clean water in one device.